The present invention relates to a method for forge or enhanced diffusion welding of two or more metal parts, wherein at least one joint is established between opposed bounding surfaces on the parts to be joined. One such method denoted Shielded Active Gas Forge Welding (SAG-FW) known from, and to a large extent defined by, U.S. Pat. Nos. 4,669,650 and 4,736,084 includes the following features:                1. The welding process consists of four main stages wherein the metal parts are:                    a. heated electromagnetically to high local temperatures,            b. brought rapidly into close contact,            c. forged together until a metallic bound is established, and            d. cooled by convection, radiation and conduction                        2. The metal parts have been carefully shaped so that there will be an advantageous triaxial state of stress as well as a high optimal closing contact pressure in the volume close to the weld during forging.        3. The parts are heated, preferably by direct high frequency resistive heating, so that the surface temperature is optimal for the material to be welded and so that the temperature gradient enhances a desirable mode of plastic deformation.        4. A reducing gas is passed between the surfaces of the parts to be welded so that oxides that are detrimental to the quality of the weld are removed before welding/fusion.        
The advantages of the method of forge or enhanced diffusion welding described in the above-mentioned patents are the high speed at which welding may be performed. The entire welding cycle may last less than a minute for easily weldable steels. Furthermore, there is no need for expensive machining or other type of trimming of the part shapes after welding since the outer surfaces of the joined parts may be almost completely flush close to the weld. There is also a potential for a high degree of process control and documentation since the temperature is much more closely controlled than for conventional welding methods.
However, in order to establish a weld of uniform quality and shape it is important to exactly control the viscoplastic deformation of the material. The viscoplastic deformation is to a large extent controlled by the temperature distribution, which may deviate from the desired one in the directions normal to and along the bevel surfaces. Also the material properties may affect viscoplastic deformation and be a cause of variability, which must be detected and compensated for.
In order to secure the highest possible weld quality it is important to make certain that the temperature of the bevel surfaces is within a certain range. A too high temperature may cause undesirable melting or excessive grain growth while a too low temperature will unavoidably lead to insufficient reduction of surface oxides and poor bounding. Undesirable material phase shifts and brittleness may also be the result of poor temperature control during heating and cooling.
The heat input and the cooling time after welding are directly related for a given part geometry and material if no special measures are implemented. A large input of heat during the heating stage of the process will produce a heated zone of large extent and cause slow cooling of the material after welding. This may be a problem particularly when welding metals that must be quenched and tempered in order to establish sufficient ductility for a given strength.
Another problem arises during welding of alloys that require artificially slow cooling after joining. After the weld has been established it is not practical with existing high frequency resistive heating technology to apply a current directly in order to prevent a sharp temperature drop. This would only cause short-circuiting with the current running from one of the electrodes to the other electrode on the same side of the part.
Hence, with conventional forge welding methods and the standard high frequency resistive heating method, it may be difficult to control the temperature and to establish optimal thermal conditions for plastic deformation, fusion and metallurgical processing at all stages and for any given material and part geometry.